There is known a motor vehicle steering system including a transmission ratio variable mechanism capable of changing a transmission ratio of an output rotation angle to an input rotation angle (for example, see Patent Documents 1 to 3).
The transmission ratio variable mechanism of the motor vehicle steering system in Patent Document 1 is provided with a wave gear mechanism, in which a wave generator, a flexspline, and a circular spline are lined up in a radial direction of a steering shaft.
The transmission ratio variable mechanism arranged in according to the Patent Document 2 is provided with a first toothed gear of which the rotation is restrained by the upper steering shaft, a fourth toothed gear of which the rotation is restrained by the lower steering shaft, and a swinging toothed gear having a rotational axis inclined to rotation axes of the first and fourth toothed gears. At the both ends of an outer race, that has toothed gear sections, of the swinging toothed gear, there are formed second and third toothed gears, and the second toothed gear is meshed with the first toothed gear and the third toothed gear is meshed with the fourth toothed gear.    Patent Document 1: Japanese Unexamined Patent Publication No. 2007-125939    Patent Document 2: Japanese Unexamined Patent Publication No. 2006-82718    Patent Document 3: Japanese Unexamined Patent Publication No. 2007-170624